1. Field of the Invention
This invention relates to a method and an apparatus for forming interconnects, and a polishing liquid and a polishing method, and more particularly to a method for forming interconnects by embedding a metal such as copper (Cu) in recesses for interconnects formed in the surface of a semiconductor substrate, and to a polishing liquid and a polishing method for use in such method and apparatus.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a semiconductor substrate, there is an eminent movement towards using copper (Cu) which has a low electric resistance and high electro-migration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in the surface of a substrate. There are known various techniques for producing such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper is deposited on the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 32A through 32C illustrate, in a sequence of process steps, an example of producing such a substrate W having copper interconnects. As shown in FIG. 32A, an insulating film 2 of an oxide SiO2 or of a low-K material is deposited on a conductive layer la formed on a semiconductor base 1 bearing semiconductor devices. A contact hole 3 and a trench 4 for interconnects are formed in the insulating film 2 by the lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the entire surface, and a seed layer 7 as an electric feed layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 32B, copper plating is carried out onto the surface of the substrate W to fill the contact hole 3 and the trench 4 with copper and, at the same time, deposit a copper film on the insulating film 2. Thereafter, the copper film 6 on the insulating film 2 is removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact hole 3 and the trench 4 for interconnects, and the surface of the insulating film 2 lie substantially on the same plane. An interconnect composed of the copper film 6, as shown in FIG. 32, is thus formed.
By the way, as shown in FIG. 33, when the copper film 6 is formed by plating on the surface of the substrate W in which a fine hole(s) 8 with a diameter d1, e.g., on the order of 0.2 xcexcm, and a large hole(s) 9 with a diameter d2, e.g., on the order of 100 xcexcm are present, the growth of plating is likely to be promoted at the portion above the fine hole 8, whereby the copper film 6 is raised at that portion, even when the effect of a plating liquid or an additive contained in the plating liquid is optimized. Further, the growth of plating with an adequately high levelling property cannot be made within the large hole 9. This results in a difference (a+b) in the level of the copper film 6 deposited on the substrate W, i.e. the height a of the raised portion above the fine hole 8 plus the depth b of the depressed portion above the large hole 9. Thus, in order to obtain the desired flat surface of substrate W with the fine hole 8 and the large hole 9 being fully filled with copper, it is necessary to provide the copper film 6 having a sufficiently large thickness beforehand, and remove by CMP the extra portion corresponding to the above difference (a+b) in the level.
This involves problems in that the large thickness of the plated film requires a prolonged time for processing by CMP in order to polish away the large amount. Increasing the rate of CMP processing to avoid the prolongation of processing time can cause dishing in the large hole.
In order to solve the above problems, it is required to make the thickness of the plated film as thin as possible, and prevent the formation of the raised and depressed portions in the plated film, despite the co-presence of fine and large holes in the surface of the substrate, thereby improving the flatness of the plated film. In this regard, when the plating treatment is carried out in an electrolytic copper sulfate bath, for example, it has not been possible to decrease both of the rise and the depression in the plated film merely by the action of the plating liquid or with an additive. It is possible to reduce the degree of rise in the plated film by temporarily using a reversed electric field as a power source, or by using a PR pulse power source during the film deposition process. This approach, however, cannot prevent the formation of depressed portions and, in addition, denatures the film at its surface portion.
Further, there is a strong demand for not resorting to CMP processing which, in general, needs a complicated operation and control, takes a considerably long processing time, and in addition, may be carried out, in general, in a separate apparatus from that of a plating treatment.
It is to be pointed out that though a low-K material, which has a low dielectric constant, is expected to be predominantly used in the future as a material for an insulating film, the low-K material has a low mechanical strength and therefore has difficulty enduring the stress applied during CMP processing. Thus, there is a demand for a method which enables the flattening of the substrate without giving stress thereto.
Further, a method has been reported which carries out CMP processing simultaneously with plating, viz. chemical mechanical electrolytic polishing. According to this method, the mechanical processing is carried out to promote the growing defect of plating, causing the problem of denaturing of the resulting film.
The present invention has been made in view of the above drawbacks in the prior art. It is therefore a first object of the present invention to provide a method and apparatus for forming interconnects which can obtain a plated film with improved flatness even when fine and large holes are co-present in the surface of a substrate, and which can carry out the subsequent CMP processing in a short time without suffering from dishing.
It is a second object of the present invention to provide a method and apparatus for forming interconnects which, while omitting a CMP treatment entirely or reducing a load upon a CMP treatment to the least possible extent, can successively carry out a series of copper interconnects-forming steps including a copper-filling step.
Further, it is a third object of the present invention to provide a polishing liquid which, when used in electrolytic polishing or chemical polishing, can polish a plated copper film formed on the surface of a substrate into a flatter film surface and can polish the surface of a substrate, in which copper and a conductive material other than copper are co-present, uniformly at the same polishing rate; and provide a polishing method which, due to the use of the above polishing liquid, can omit a CMP treatment entirely or can reduce the load from a CMP treatment to the least possible load.
In order to achieve the first object, the present invention provides a method for forming interconnects, comprising providing a substrate having fine recesses formed in a surface thereof plating the surface of the substrate in a plating liquid and electrolytic etching the plated film formed on the surface of the substrate in an etching liquid.
This method, when applied to a substrate having fine holes and large holes in the surface, promotes a bottom-up growth of plating in a large hole by carrying out plating in a plating liquid having a high levelling property, whereby it is possible to fill the large hole with a thinner plated film. Concomitantly with the bottom-up of plating in a large hole, the raised portion of plating above a fine hole becomes thicker. The raised portion can be selectively removed by the electrolytic etching. The above method can thus improve the flatness of a plated film.
In the present invention, the plating is carried out in a plating liquid for exclusive use in plating, and the etching in an etching liquid for exclusive use in etching, so as to prevent the plated film from deteriorating.
The etching liquid may preferably contain at least one additive selected from the group consisting of an additive which forms a complex compound or an organic complex with the metal of the plated film and an additive which can lower the corrosion potential of the metal of the plated film. The additive for forming the complex compound may specifically be pyrophosphoric acid or aminocarboxylic acid (e.g. glycine). The additive for forming the organic complex may be ethylenediamine, EDTA, DTPA, iminodiacetic acid, TETA, or NTA. When the plated film is a copper film, the additive which can lower the corrosion potential of copper includes thiourea and its derivatives.
A waveform of current flowing in the electrolytic etching may be, for example, a pulse waveform or a PR pulse waveform. Such waveforms can improve diffusion of the additive contained in the etching liquid.
The apparatus for forming interconnects of the present invention comprises a plating section for holding a plating liquid and plating a surface of a substrate having fine recesses formed in the surface thereof in the plating liquid and an etching section for holding an etching liquid and electrolytic etching the plated film formed on the surface of the substrate.
According to the apparatus, plating and electroetching can be carried out in a successive manner. Further, repeating the plating and electroetching treatments can further improve the flatness of the plated film.
The etching section may include, for example, a substrate holder for holding a substrate with its surface downward, a cathode plate immersed in the etching liquid and located facing the lower surface of the substrate held by the substrate holder, and a relative movement mechanism for allowing the substrate held by the substrate holder and the cathode plate to move relatively. The relative movement between the substrate and the cathode plate prevents a plated film from being locally etched excessively to worsen the flatness of the film.
The relative movement mechanism may comprise a substrate-rotating mechanism for rotating the substrate and a cathode plate-moving mechanism for rotating, reciprocating, eccentrically rotating the cathode plate, or making a scroll motion of the cathode plate.
This mechanism makes the velocities of the substrate at its various points relative to the cathode plate closer to one another so as to make the flow conditions of the etching liquid between the various points of the substrate and the cathode plate uniform, thereby avoiding the generation of a singular point in the flow of etching liquid. The distance between the cathode plate and the substrate (anode) should preferably be made as small as possible mechanically, and is preferably 1.0 mm or less, more preferably 0.5 mm or less.
According to a preferred aspect of the present invention, the apparatus further comprises a plurality of grooves extending over the full length of the cathode plate in the surface thereof, and a plurality of etching liquid feed holes formed in the cathode plate for feeding the etching liquid to the grooves, the plurality of etching liquid feed holes communicating with the grooves.
During the electroetching, the etching liquid is fed from the grooves formed in the surface of the cathode plate to between the two electrodes, i.e. the cathode plate and the substrate, while particles floating in the etching liquid is allowed to pass through the grooves to the outside by the action of centrifugal force. This makes it possible that a fresh etching liquid is always present between the electrodes. The grooves may preferably be formed in parallel or in a lattice arrangement so as not to make a difference in current density between the center and the periphery of the cathode plate and, in addition, to allow the etching liquid to flow smoothly to the outside.
The substrate holder may be constructed to hold the substrate in a vacuum attraction manner or in an electrostatic chucking manner. Such a substrate holder can hold the substrate by attracting the entire surface of the substrate, thereby absorbing undulations present in the substrate, so that the substrate holder can be held with a flattened state.
The cathode plate may be composed of a material having a poor adhesion to copper. When the electroetching is carried out, for example, to a plated copper film by using a cathode plate made of e.g. titanium whose oxide shows poor adhesion to copper, the dissolved copper ions are precipitated onto the cathode plate side but the precipitate is immediately released from the cathode plate to float as copper particles in the etching liquid. The etching liquid containing the floating copper particles is allowed to flow to the outside. The etching can thus be carried out without suffering from the deterioration with time of the surface flatness of the cathode plate. Further, the generation of hydrogen gas during the etching can be prevented. The etching can thus provide the etched surface with excellent flatness.
In order to achieve the second object, the present invention provides an apparatus for forming interconnects by forming a copper film on a surface of a substrate to fill copper into fine recesses formed in the surface of the substrate, comprising a housing, a transport route provided in the housing for transporting the substrate, and a copper-plating section, an electrolytic or chemical polishing section, and an annealing section which are disposed along the transport route.
According to this apparatus, the flattening process after copper plating is carried out mainly by means of electrolytic or chemical polishing. Thus, the apparatus can omit a CMP treatment entirely or reduce a load upon a CMP treatment, and can successively carry out a series of flattening steps including annealing in the same housing.
A cleaning section may be provided in the housing for cleaning the substrate.
At least two of the electrolytic or chemical polishing sections may be provided for carrying out a first-stage electrolytic or chemical polishing and a second-stage electrolytic or chemical polishing. This enables such two-stage electrolytic or chemical polishing treatment to the surface of copper that the rate of polishing or the polishing selectivity to the base is made different between the first and the second stages so as to obtain a flatter copper surface, or that the surface of copper is polished in the first stage, and in the second stage, the exposed copper and other conductive materials (e.g. TaN) are polished evenly at the same polishing rate.
The apparatus may be provided a cap-plating treatment section for forming a protective film which selectively covers and protects the exposed surface of copper interconnects. The cap-plating treatment for protecting the exposed surface of copper interconnects by the selective coating of a protective film thereon can thus be carried out successively in the same housing.
The present invention provides a method for forming interconnects by forming a copper film on a surface of a substrate to fill copper into fine recesses formed in the surface of the substrate, comprising plating the substrate with copper to form the copper film on the surface and to fill copper into the fine recesses of the substrate, electrolytic or chemical polishing the surface of the substrate having the copper film thereon in a polishing liquid and annealing the substrate in such a state that the copper film remains on the entire surface of the substrate, after the polishing.
After the annealing treatment, the substrate may be subjected to a CMP treatment, and the treated substrate may then be subjected to the above described cap-plating treatment to selectively cover the exposed surface of copper interconnects with a protective film. The above manner of first carrying out the wet treatments, followed by the dry treatments, has the merit that the wet treatment sections and the dry treatment sections can be arranged in separate divisions in an apparatus. It is however possible to follow the sequence of platingxe2x86x92annealingxe2x86x92electrolytic or chemical polishingxe2x86x92CMP.
The present invention further provides a method for forming interconnects by forming a copper film on a surface of a substrate to fill copper into fine recesses formed in the surface of the substrate, comprising plating the substrate with copper to form the copper film on the surface and to fill copper into the fine recesses of the substrate, annealing the substrate having the copper film thereon and electrolytic or chemical polishing the surface of the substrate in a polishing liquid after the annealing. This method can omit a CMP treatment entirely.
The substrate may have a cap-plating treatment applied there to to selectively cover the exposed surface of the copper interconnects with a protective film after the polishing.
In order to achieve the third object, the present invention provides a polishing liquid for use in electrolytic or chemical polishing of copper by immersing therein a substrate having fine recesses in a surface thereof which are filled with copper by forming copper film, comprising at least one inorganic acid and/or an organic acid capable of dissolving copper and at least one viscosity-increasing agent selected from the group consisting of polyhydric alcohols, high-molecular weight polyhydric alcohols and alkylene glycol alkyl or aryl ethers.
The polishing liquid, when used in the electrolytic or chemical polishing of the surface of a copper film formed on a substrate, can enlarge the diffusion layer on the substrate in which a copper complex is present, and therefore can raise the polarization potential and suppress the conductivity of the entire surface of the substrate in the liquid, thereby suppressing the dissolution of copper over the entire substrate surface and/or the movement of copper ions in the liquid and making the surface not sensitive to a minute change in current density, whereby the polished surface endured with high-flatness can be obtained. In this connection, it has been found that the enlargement of the diffusion layer, the rise in polarization potential and the suppression of conductivity depend largely on the viscosity of the polishing liquid used.
Examples of the polyhydric alcohols include ethylene glycol, propylene glycol and glycerin. Examples of the high-molecular weight polyhydric alcohols include polyethylene glycol and polypropylene glycol. Examples of the alkylene glycol alkyl or aryl ethers include ethylene glycol ethyl ether, ethylene glycol methyl ether, ethylene glycol propyl ether, ethylene glycol phenyl ether, propylene glycol ethyl ether, propylene glycol phenyl ether and dipropylene glycol monomethyl ether.
The polishing liquid preferably has a viscosity of 10 cP (0.1 Paxc2x7s) or more and a conductivity of 20 mS/cm or lower.
It is preferred that the polishing liquid further contains an additive which can adhere to the surface of copper and electrically and/or chemically suppress the dissolution of copper. The use of the polishing liquid containing such an additive in the electrolytic or chemical polishing can provide the polished copper surface with improved flatness. Further, when copper and other conductive materials (such as TaN) are exposed on the surface of a substrate, the electrolytic or chemical polishing of the substrate with the use of the additive-containing polishing liquid can polish the copper and the other conductive material (such as TaN) evenly at the same polishing rate. Specific examples of the additive may include imidazole, benzimidazole, benzotriazole and phenacetin.
The polishing liquid may preferably contain a basic liquid or an additive which forms a strong complex with copper or promotes the formation of a passivated film on the surface of copper. The use of the polishing liquid containing such a basic liquid or an additive can provide the polished copper surface with improved flatness.
Further, when copper and other conductive materials (such as TaN) are exposed on the surface of a substrate, the electrolytic or chemical polishing of the substrate with the use of such polishing liquid can polish the copper and the other materials (such as TaN) evenly at the same polishing rate. Chromic acid may be mentioned as an example of the basic liquid for promoting the formation of a passivated film on the surface of copper. EDTA and quinaldin may be mentioned as examples of the additive, and pyrophosphoric acid as an example of the basic liquid, for forming a complex with copper.
The present invention provides a method for polishing a substrate having fine recesses in a surface thereof which are filled with copper by forming copper film, comprising electrolytic or chemical polishing the surface of the substrate, where only copper is exposed thereon, in a polishing liquid in which the dissolution of copper is suppressed and electrolytic or chemical polishing the surface of the substrate, where only copper is exposed, or copper and a conductive material other than copper are exposed, in a polishing liquid in which the dissolution of copper is further suppressed.
According to this method, the unnecessary portion of plated copper film can be remove by the electrolytic polishing into a flat surface, and the flatness can be improved by the subsequent electrolytic or chemical polishing. Alternatively, the first electrolytic polishing to remove the unnecessary copper is allowed to proceed until a conductive material other than copper (such as TaN) becomes exposed on the surface, and the exposed material (such as TaN), together with the exposed copper, can then be polished away at the same rate by the second electrolytic or chemical polishing into a flatter surface. The polishing method of the present invention can thus omit a CMP treatment entirely, or reduce a load upon a CMP treatment to the least possible extent.
Copper remaining on the surface of the other conductive material is removed by electrolytic or chemical polishing. The removal of such copper can avoid a rise in polishing rate of copper, which would be caused if the copper would remain unremoved, in the subsequent electrolytic or chemical polishing.
The other conductive material, not having been removed, may be removed. After the electrolytic or chemical polishing, the other conductive material, not having been removed by the polishing and remaining on the insulating film, e.g. a SiO2 oxide film or a film of low-K material, may be removed without resorting to a CMP processing.
The copper and/or the other conductive material remaining on the surface of the substrate may be removed either by passivating only the surface of the copper and preferentially electrolytic or chemical polishing the other conductive material, or by passivating the entire surface including the copper and the other conductive material, and composite electrolytic polishing said entire surface.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.